Lately it seems like the life on my macbook has been getting dramatically shorter over the past couple of months. Just today I opened it up, and overnight w/out being plugged in it went from 99% to 54% . Furthermore, the thing will just abruptly drop somewhere around 28-30% to 0% without warning . Is it time for a new battery? I've had this battery since I got it--back in May of '07.

If it's time to get a new battery, what's the most cost-effective way of getting one? I couldn't find one on the 'egg but I did see some "white macbook" batteries on eBay. How do I know which is the right battery for me? (intel C2D 65nm merom core on Napa chipset, GMA950)

You don't need the "right" battery; you need a battery that fits (and all plastic Macbooks have used the same size battery IIRC) and provides the right voltage (which is probably the same across all Mac laptops, though I don't know).

However, a battery dying that quickly after less than 1.5 years is a bit odd -- how many charge cycles have you put it through?

I'd take it to an Apple Store -- I think that they say your batteries should last through 300 cycles without appreciable damage (generally meaning at least 80% capacity should remain at 300 cycles). It isn't under warranty but if you take the trouble to go see them the Geniuses are apparently pretty forgiving.